Standard 5 pin mini-USB connector has pin X connect to ground (pin 4)
through a 200K ohm resistor (Note: The normal USB connector has only 4
pins, the mini-USB has 5 pins). To place Nuvi into charging mode, the
200K ohm resistor must be changed to 17.3K ohm. (also successful with
22K ohm resistor from Radio Shack, also a cheap 1/8W, 5%, 15k ohm
resistor will work just fine!)

Garmin Oregon requires a resistor of at least 47K ohm for running in
external power supply mode (100K ohm also works). Smaller values are
working, too, but Oregon then first shows a message that an unsupported
accessory was found.

GPSMAP 62:

pin X not connected – data cable, GPSMAP 62 connects to PC as removable drive.
If it fails because it is connected to your car battery, it switches to external power supply mode after several secondes.